1. Field of the Invention
The present invention relates to a surgical implant in the nature of a subtalar implant and the method of use thereof for implantation to correct podiatric disorders.
2. Description of Related Art
A subtalar joint (STJ) arthroereisis is provided as well as methods of use thereof for the purpose of correcting podiatric disorders such as various types of flat foot conditions relating to the STJ. The STJ arthroereisis is surgically implanted in the sinus tarsi. The sinus tarsi is a conical-shaped cavity located between the anterosuperior surface of the calcaneus and the inferior aspect of the neck of the talus. Opening laterally, the sinus tarsi is anterior to the fibular malleolus and terminates posteromedially directly behind the sustentaculum tali. Once implanted, the STJ arthroereisis functions primarily to preserve the STJ while also to limit excessive valgus motion and calcaneal eversion and to limit anterior and plantar migration of the talus.
The development of STJ arthroereisis for correcting podiatric disorders including flat foot conditions dates back to 1946. The original STJ arthroereisis incorporated a theory of using a bone graft to elevate the sinus tarsi and to limit pronation of the subtalar joint. Procedures built upon the bone grafting techniques to eliminate pronation by positioning a bone graft or a custom carved silicone wedge inside the sinus tarsi. The bone grafts and silicone wedges were further developed into a stemmed polyethylene block known better as the STA-peg, which was further modified through time. The bone grafts, the silicone wedges, and the stemmed polyethylene block were later replaced with modern, threaded implants for surgical implantation within the sinus tarsi. For example, the Maxwell-Brancheau Arthroereisis (MBA) implant is a cannulated, soft-threaded screw-shaped cylindrical prothesis manufactured from a premium titanium alloy. Newer generation threaded implants developed as a conical subtalar implant (CSI), which is a cannulated, soft-threaded conical prothesis, and a domed-stemmed subtalar implant (DSI), which is a cannulated, domed and stemmed prothesis, both of which are manufactured from a premium titanium alloy.
While initially described as a simple bone graft to elevate the floor of the sinus tarsi, STJ arthroereisis has developed into a modern generation of implants. The outcomes anticipated from STJ arthroereisis with the modern implants include a decrease in frontal plane heel valgus, an improved medial arch height while bearing weight, a decreased pronatory motion of the STJ, a decreased mid-tarsal joint (MTJ) inversion and eversion, and a spared STJ inversion. Additionally, STJ arthroereisis with the modern implants provide a re-alignment of previously anteriorly displaced cyma line, decreased talo-navicular joint (TNJ) subluxation and talar declination, and an increased calcaneal inclination.
Several problems are associated with the modern implants for STJ arthroereisis. The features of modern implants lack positioning control capabilities for use during implantation surgery, which requires precise and controlled manipulation of the implant for final positioning. Additionally, the geometric shape of the modern implants is either cylindrical or conical. Modem cylindrically-shaped implants provide poor anatomical fit with the STJ structure. Modem conically-shaped implants may offer a slightly better fit with STJ anatomical structure than cylindrically-shaped implants, but require apertures along the softened thread surface for post implantation osseous tissue growth to stabilize the implant.
Accordingly, the subtalar implant of the present invention overcomes the problems associated with modern implants for STJ arthroereisis. In accordance with the present invention, the subtalar implant is capable of threaded engagement with a positioning element, which is used for positioning control and manipulation of the subtalar implant during surgical implantation. For assistance in final positioning for implantation, the subtalar implant is cannulated to receive a guide rod to facilitate proper positioning of the subtalar implant with a cannulated driver. Once implanted, the subtalar implant of the present invention provides a superior anatomical fit with the STJ anatomical structure, without the need for osseous growth indentations to anchor the subtalar implant.